How to choose a grow light

Factors to Consider When Choosing an LED Grow Light

The time has finally arrived and you are ready to make your dream grow light purchase. After months of data checking, spectrum comparisons and efficacy comparisons between different brands, you feel you are one step closer to picking the most suitable grow light for you. 

Before biting the bullet and investing your hard earned wages, you may still have several questions in the back of your mind such as:

• What size grow area do I need?
• How many plants to grow in your grow tent or grow area?
• How much yield will I get from my grow space?
• What grow light wattage you need for your space?
• What is the best grow light spectrum?
• Features to look for in a grow light
• How much should I pay for a high quality grow light?
• What grow light features to look for?

Get ready to learn all you need to know about choosing a grow light and find out exactly what is the best grow light for you.

What size grow area do I need?

As a first time grower, you may not be aware of what a grow area even means, let alone how much space you need. A grow area simply refers to the space where you will hang your grow light.  You may have identified a space in your house or shed to put a grow tent or have a cupboard or small room you want to convert for growing. 

Most growers will use a grow tent and a good place to start is looking at the most popular grow tent sizes and the approximate number of plants you can grow in Figure 1 below

How many plants can you fit in your grow space?

Now we have figured out the grow space size, the next question is how many plants can I fit inside? One important factor that should be considered is the more plants you put into a grow area the less each plant has to grow to fill the canopy and utilise all of the light energy available. However the downside is requiring more seeds, pots etc. Growing a larger quantity of smaller plants also gives you some safety in numbers should some plants die. 

Growing a smaller quantity of larger plants requires less seeds, pots etc. However larger plants require much longer to fill the grow space so the time to harvest will also be longer. 

Autoflower plants tend to grow faster and are usually smaller than photoperiod plants with a long vegging time. To help decide on which type of plant to grow please see our guide the recommended PAR levels for photoperiod and autos.

So what yield can I expect from my light and grow space?

This is a very important consideration for a first time grower. There are many elements to the yield equation that can make a significant difference, which are listed below.

Grow area

The yield for a given grow area is a good general guide to estimating your potential yield in Figure 2 above. 

Grow light wattage 

How much power and PAR the lights produce plays a massive role. The amount of watts that the grow light consumes is a very good indicator of how much light will be delivered to the plants. LED grow lights are very efficient and will deliver a much higher yield per watt than High Pressure Sodium, ceramic metal halide or fluorescents.

However, where your grow performs in the range of outcome is dependent on a number of factors.

Plant genetics 

Not every plant has the ability to produce the same yield. The range of yield per plant depends on the genetics and final size of the plant. Photoperiod plants vegged for a long time will be at the upper range and smaller photoperiod plants or autoflowers will be at the lower end of the range. 

Photoperiod vs autos 

The first point to note is that Photoperiod plants take a lot longer to grow to harvest at initial 18/6 light cycle and then 12/12. Autoflower plants can be grown in a 20/4 light cycle, receive more light each day and mature to harvest sooner. Discover all you need to know about the recommended PAR levels for photoperiod and autos.

Growing medium 

The yield per watt of grow light  is heavily dependent on the growing medium. In general Hydro is most efficient, coco next and soil least efficient (this is generally true, all other things being equal). 

Environment control 

Dialling in your temperature and humidity is key. Plants thrive under specific environmental conditions during various stages of their life cycle. It is essential to have consistent lights on and lights off temperature and humidity, with plenty of fresh air blowing around the base and tops of the plants. 

Carbon dioxide supplementation

Carbon dioxide levels in the atmosphere are around 400 ppm. Indoors plants are able to grow at a faster rate with co2 levels between 1,000 - 1,200 ppm. Using carbon dioxide can increase growth rate and yield by up to 30%. However, it is expensive and complicated to setup, particularly for smaller home growers.

A grower’s experience 

We all have to start somewhere and a great grower can hit consistent yields. However, we have spoken to a lot of new and experienced growers to determine the general range of yield potential. We have produced a guide for expected yield by grow area, grow light wattage and by plant in the below chart

How many watts of power will my grow space use?

For many of us, this will be one of the toughest parts of buying your grow light. You have to ask yourself how much does it cost to run a grow light? 

Before you base everything off the number of watts, you should think about the Photosynthetically Active Radiation (PAR) it delivers to the plants. PAR is basically a technical way of saying how much useful light the plant canopy receives.

To maximise the yield from your grow space it is necessary to deliver a high average PAR to the plant canopy. However there is a reducing rate of return as you increase the average PAR above a certain level. 

You may think it all sounds a bit too technical and certain parts are going over your head, but if you look at Figure 3 below, you can see there is a correlation between PAR and the growth rate of the plants.

Figure 3

Photoperiod and Autoflower plants

Photoperiod plants require 18 hours of light and 6 hours of dark in the early vegetative stages. To force photoperiod plants to flower the light schedule must be switched to 12 hours on and 12 hours off. Figure 4 below is a chart explaining the number of watts required for a grow space with photoperiod plants.

Autoflowering plants, as the name suggests, will begin to flower around week 4 with no light schedule change. Autoflowering plants can be left to grow from 18-24 hours of light for the whole grow. Autoflowers have a longer light cycle. The optimum PAR level is lower than for photoperiod plants. Figure 5 below is a chart explaining the number of watts required for a grow space with autoflowering seeds. 

For started growers we recommend an average PAR of about 900 µmols/m²/second for photoperiod plants and 550 µmols/m²/second for Autoflower plants.  

For the most popular grow tent sizes figure 6 below lists the recommended grow light wattage for both photoperiod and autoflower growers. We would encourage you to find out more in our blog post all you need to know about the best PAR intensity for plants.

One of the most important pieces of information to look for in a grow light specifications is a PAR map or PAR chart. The PAR chart will display the PAR intensity delivered by the grow light in its specified area. You should be able to see the average PAR intensity across the grow area. You should also check for the uniformity of spread across the target area. Many light swill deliver high PAR intensity in the centre but there will be a big drop off at the edges.

Figure 7 is a PAR chart of the PAR intensity the ARAY 5X5 fixture delivers at the recommended hanging height.

Figure 7

What is the best Grow Light spectrum?

You may be planning your first grow or you may have tried different types of grow bulbs ranging from compact fluorescent, high pressure sodium and metal halide. You may be asking yourself what colour light is the best for growing plants? Well, let's break down the topic of spectrum so you can easily understand what it is and why it is so important.

Using LEDs allows us to tailor the grow light spectrum that we want to deliver to the plants. However you may notice that most grow light manufacturers use a very similar LED selection to achieve the best full spectrum output to grow from seed to harvest. This is generally achieved by using a mix of cool and warm white LEDs and 660nm deep reds. The mixture of white LEDs delivers a neutral white of about 4000K CT which has about 12% to 15% blue. 

The reason grow light manufacturers use this common spectrum recipe is:

• Blue is critical for short internodal distances (distance between branches) and above 5% will deliver the desired short and dense growth.

• The 660nm reds are very efficient in terms of electrical efficiency (PAR output per watt) and also photosynthetically efficient. Photosynthetic efficiency means red photons cause photosynthesis at a higher rate than green and blue photons.

• 660nm reds are  relatively expensive but add a 'turbo charge' to the overall system efficiency of the grow light in terms of PAR output per watt.

• Some growers choose to add Far red to the grow light spectrum. There are limited benefits and some risks to adding Far red in the spectrum. Find out more about far red and plant growth.

There is little difference between most grow light spectrum on the market and for good reason, the established standard spectrum is close to optimal for growing plants from seed to harvest. If you want to maximise yield and potency delivering high intensity PAR levels to your plant canopy is the best way to achieve this goal.

This grow light spectrum in fig 8 shown below is perfect to grow healthy plants. There are arguments that adding Far Red to promote flowering and UVA to stimulate higher potency but none of these theories are proven. 

Figure 8

Features to look for in a grow light

Before you make your final decision, there are a few more things to consider. One of those is how do you know what a good grow light even looks like or feels like? With so many different shapes, sizes and versions available, it can be overwhelming sometimes to know who is the best and why. Below are a few factors to consider that will help make the decision making side of things much easier. 

1. It is important to see how uniform and intense the PAR is to compare to other light fixtures you are considering. Only consider buying a grow light with a published PAR map showing the PAR intensity delivered to the plant canopy.

1. Does the grow light have dimming capability built in and the option of using an external controller and daisy chain multiple fixtures in larger grow rooms.

1. Can the LED driver be hung outside the grow room to reduce heat in the grow room?

1. Are the LEDs protected from dirt and moisture with acrylic or silicone coating?

1. Is the warranty at least three years and what are the terms i.e. is delivery, parts and labour included?
2. Does the manufacturer or brand have a good reputation for service delivery? Check independently verified reviews and online growers forums.

Learn more about grow lighting basics  and really fine tune your knowledge before making a purchase.

How much does a high quality grow light cost?

At this point in the article, you are getting your head around the technical side of things, understand LED grow lights are superior. Now all you need to know is what can you expect to pay for a top quality grow light? The easiest way to look at that, is to base it off the initial investment combined with the electricity running costs. 

You can see that generally the more you pay for a light the efficiency is higher and it will pay you back over time as the electricity running costs are lower. MIGRO LED lights come in different sizes for various grow spaces, so to find out which light is most practical for you, please check out our Grow light store 

MIGRO top tip  - Use a seedling light for 33% more yield!

Did you know that one of the best ways to achieve the ultimate yields from your indoor grow space, is by using a seedling light? Use a seedling light for 33% more yield and become the most efficient grower possible, whilst reducing your energy costs and electric bills.

Our final thought…

New grow lights are being released all the time so it is difficult to know the long term reliability of most models on the market. However you can research the brand and model on forums and review sites. Customers of the brand will give you some indication of the company's reliability and service levels so you can avoid companies with bad reputations. 

There are many good grow light manufacturers you can choose from so take the time and effort to research them. Be sure to check out the Migro light reviews and grow light science and news for more detailed information, giving you the most advantage possible before making a purchase.